My bad, I didn't realize Scotty had already tried to fix the thread seal.
1 demerit for me

 

It’s good. Machine shop said it’s not the threads, the problem is in the flatness where the plug tightens down to.
But of course it’s still leaking around the plug, so not the solution!!!

 

The angle of the spot face cut they made has to be perpendicular to the threads
Wonder how they measured that

 

We made a tool from an end mill that drops in the the thread and uses the thread length as a center guide its 2 pieces - a drilled insert ( pilot ) goes in then the tool so the threads are never in danger of being hurt

Indian - 45 and big twin share a 14mm thread and again some are 18mm < those are easy as a pilot is standard for converting to 14mm plug that most all flatheads have - the guy using the bridge port must be good at what he does or you end up doing nothing but reducing the distance

 

What year Harley? Show a picture of the bike, the head were the shop cleaned up the port. The sparkplug. Put some high spot blue on the and see if the plug rings it 36. If threads were not square with face, it's pretty involved getting them that way.

I would thread a plug on my lathe to a snug fit on the thread and a straight section to last word indicator to get it square.

Is your plug not a crush gasket ? The old school 45 bevel?


Now it is normal in 10,000 miles US to see black up the threads on a FI even. Aluminum when hot outgrows steel.

Class 2B threads have more then enough clearance for a crush gasket or 45 seat to square up and seal .

 

They may have used a tool similar to this Time-cert tool. The long piece threads into the head and will determine the proper angle for the sealing surface. The Wider piece slides over the first piece and cuts the sealing surface.